July 25, 2014

Is The Earth Facing A Modern-Day Extinction Event?

After nearly 3.5 billion years of evolutionary trial and error, the biodiversity of our planet is the highest it has ever been. An international group of scientists warns, however, that it may be reaching a tipping point.

The team — consisting of scientists from Stanford University, University of California, Santa Barbara, Universidade Estadual Paulista in Brazil, Universidad Nacional Autonoma de Mexico, the Natural Environment Research Council Centre for Ecology and Hydrology in England, and University College London — conducted a review of scientific literature and analyzed data. Their findings, published in Science, indicate that the loss and decline of animals is contributing to what appears to be the early days of the planet's sixth mass biological extinction event.

Species are becoming extinct at an alarming rate. For example, more than 320 terrestrial vertebrates have become extinct since 1500, and the remaining species show an average 25 percent decline in abundance. Invertebrate animals face a similar challenge.

The previous five extinction events have been precipitated by natural planetary transformations, or catastrophic asteroid strikes — such as the dinosaur "killer" meteor that created the Chicxulub Crater in the Yucatan Peninsula, Mexico. The event this study predicts is strongly associated with human activity. Rodolfo Dirzo, professor of biology at Stanford and a senior fellow at the Stanford Woods Institute for the Environment, calls this an era of "Anthropocene defaunation."

In the vertebrate kingdom, scientists estimate that 16 to 33 percent of all species are globally threatened or endangered. Megafauna (large animals such as elephants, rhinoceroses, polar bears and others) are in the highest rate of decline, following a trend set in previous extinction events.

Larger animals need larger habitats to maintain viable populations. Their population growth rates tend to be lower than smaller animals, and they produce fewer offspring. Their oversized bodies and meat mass also make them inevitable targets for human hunters.

Megafauna, as a group, represent a small percentage of the animals that are threatened. Their loss, however, would have a trickle-down effect that would damage the stability of other animal species, including humans.

To understand this trickle-down effect, prior studies conducted in Kenya isolated patches of land from zebras, giraffes and elephants. The researchers observed how the ecosystem reacted to the loss of its largest species. The first major change was an inundation of rodents. Grasses and shrubs — food sources for the larger animals — grew more abundantly and the rate of soil compaction decreased. Other effects include an increase in seeds and shelter, and a decrease in predation risks.

The lack of predation allows the rodent population to double, bringing with it an abundance of disease-carrying ectoparasites.

"Where human density is high, you get high rates of defaunation, high incidence of rodents, and thus high levels of pathogens, which increases the risks of disease transmission," said Dirzo in a recent statement. "Who would have thought that just defaunation would have all these dramatic consequences? But it can be a vicious circle."

The study findings also revealed a disturbing trend in invertebrate defaunation. As the human population has doubled in the past 35 years, invertebrate populations (beetles, butterflies, spiders, and worms) have declined by nearly 45 percent. Loss of habitat and global climate disturbances have driven this loss of invertebrates, which, in turn, could drive a trickle-up effect.

Approximately 75 percent of the world's food crops are pollinated by insects, representing an estimated 10 percent of the economic value of the global food supply. In ecosystem productivity, insects play vital roles by participating in nutrient cycling and decomposing organic materials. Just in the US, pest control by natural predation has an estimated value of $4.5 billion annually.

The researchers do not believe there is a simple solution. Though the immediate reduction of habitat change and overexploitation would help, it could not be a "one-size-fits-all" solution. Each region would need a plan tailored to fit its needs. Dirzo hopes that raising awareness of the human-driven mass extinction and the consequences we face because of it will help spur change.

"We tend to think about extinction as loss of a species from the face of Earth, and that's very important, but there's a loss of critical ecosystem functioning in which animals play a central role that we need to pay attention to as well," Dirzo said. "Ironically, we have long considered that defaunation is a cryptic phenomenon, but I think we will end up with a situation that is non-cryptic because of the increasingly obvious consequences to the planet and to human wellbeing."